This invention relates to magnetic resonance(MR) imaging apparatus. More specifically, this invention relates to a MR imaging apparatus for quantifying a microscopic motion in a tissue of a human body.
As is now well known, the MR phenomenon can be excited within a sample object, such as a human patient, positioned in a homogeneous polarizing magnetic field, by irradiating the object with radio frequency energy at the Larmor frequency.
The technique for quantifying a microscopic motion, especially intravoxel incoherent motion(IVIM) by using MR phenomenon, has worked, for example, by Stejscal E. O., et.al. "Spin diffusion measurement: spin echoes in the presence of time-dependant field gradient" J Chem Phys; 43:288-292, 1965, D. Le Bihan et.al. "MR imaging of Intravoxel Incoherent Motions: Application to diffusion and perfusion in neurologic disorders", Radiology, 161: 401-407, 1986 or "Separation of Diffusion and Perfusion in Intravoxel Incoherent Motion(IVIM) MR imaging", Radiology, 168: 497-505, 1988.
Intravoxel incoherent motion(IVIM) is a term that designates the microscopic translational motions that occur in each image voxel in MR imaging.
In biologic tissues, these motions include molecular diffusion of water and microcirculation of blood in the capillary network(perfusion).
In operation of the MR imaging apparatus for imaging the diffusion or the perfusion, a motion probing gradient(MPG) in large amplitude is applied to a patient so as to probe a micro motion in tissue of the human body of a patient, for example.
When the MPG pulse is applied to the patient, a phase shift does not generate in a static spin of the patient but useful phase shift generates in a moving spin only because of a flow of the moving spin.
Therefore, the phase shift of the moving spin can be emphasized and the diffusion and the perfusion can be quantified.
However, the moving spin includes other spins, for example, a respiratory motion, cardiac motion or a cerebrospinal fluid(CSF) motion.
These moving spins cause the phase shift and therefore a "ghost" or a motion artifact appears in the monitor, which is harmful for the imaging of the diffusion or the perfusion.
To reduce the motion artifact, various imaging methods have developed in the ordinary body imaging, for example, a signal averaging method, an ECG(electro-cardiograph) gating method or an EPI(Echo Planar Imaging) method.
The signal averaging is frequently employed in MR imaging to improve signal-to-noise ratio in the body imaging.
However, in the averaging method, acquisition time is directly proportional to the number of averages and so a significant time must be required.
In the ECG method, many electrodes for gating must be attached to a patient body and there must be a long time for preparing the imaging.
Furthermore in a patient having arrhythmia, a repetition time of excitation(TR) are varied and other ghosts may be generated and to a large CSF motion, the ECG method is not effective for reducing the motion artifact.
The EPI method has not seen much usage due to a large number of problems, including hardware problems, such as the large gradient amplifier that must be provided.
Furthermore, a signal which includes the diffusion signal only and does not include the perfusion signal, can not be acquired.